1. Field of the Invention
The present invention relates to glass compositions suitable for use in stabilization of radioactive, hazardous, or mixed waste, having relatively low melting points and relatively high amounts of lithia (Li.sub.2 O), to methods of making these glass compositions, and to methods for using the compositions to immobilize waste materials.
2. Description of the Related Art
Various hazardous, radioactive, and mixed (both hazardous and radioactive) wastes, including heavy metal wastes such as lead paint and contaminated soils, require stabilization in solid forms that meet regulatory disposal criteria promulgated by government agencies like EPA and NRC. As discussed below, these wastes originate from a variety of sources, and consequently can exist in a variety of forms, including aqueous waste streams, sludge solids, mixtures of aqueous supernate and sludge solids, combinations of spent filter aids from waste water treatment and waste sludges, supernate alone, incinerator ash, incinerator offgas blowdown, or combinations thereof, geological mine tailings and sludges, asbestos, inorganic filter media, cement waste forms in need of remediation, spent or partially spent ion exchange resins or zeolites, contaminated soils, lead paint, etc.
Many industrial processes generate hazardous wastes in the form of aqueous waste streams, sludge solids, aqueous supernate, incinerator ash, incinerator off gas condensate, and so forth. Waste treatment processes may themselves generate secondary hazardous wastes. For example, solids can be filtered from an aqueous waste stream by passing the stream through filter aids, such as perlite (PERFLO) or diatomaceous earth filters. The spent filter medium is impregnated with the materials removed from the waste stream, such as heavy metals and other hazardous or radioactive substances. The spent filtration wastes must themselves be treated and stabilized before disposal. As used herein, the term "hazardous waste" includes wastes containing substances commonly recognized as hazardous, including but not limited to, chemical wastes, radioactive wastes, mixed chemical and radioactive wastes, heavy-metal-containing wastes, and hazardous organics.
Stabilizing hazardous wastes using currently available technology is expensive and requires enormous resources of equipment and personnel. Stabilization processes must be operated within guidelines established under the Resource Conservation and Recovery Act (RCRA), and the stabilized product must meet stringent state and federal standards. In the case of radioactive or mixed wastes, the stabilized wastes must often be stored for long periods of time waiting for decay of the radioactive components before transportation to an approved underground repository. Minimizing the waste volume is important in minimizing storage, transportation, and final disposal costs.
Incinerators are often used to destroy the hazardous constituents of solid and liquid wastes, as well as municipal garbage. Byproducts of incineration include bottom ash, aqueous incinerator offgas condensate (blowdown), or mixtures of ash and offgas condensate, all of which may contain residual hazardous and/or radioactive substances.
Radioactive waste may be further categorized into high level waste (HLW) and low level waste (LLW). High level waste is generally generated by reprocessing of spent nuclear fuel and other irradiated material, weapons production, research and development, etc. High level waste resulting from fuel reprocessing generally is an acidic, highly radioactive, and heat producing liquid that is generally either calcined to a dry, granular solid or neutralized, dehydrated, and stored as a damp salt, sludge, and supernate liquid. Low level waste generally contains more radioactivity than is allowed for municipal disposal, but are not sufficiently radioactive to produce substantial amounts of heat. Low level waste typically includes contaminated soil, clothing, gloves, resins, waste sludges, etc.
Hazardous wastes may be solidified by vitrification (incorporation into a glass matrix) or cementation. In typical cementation processes, cement-forming materials are added to the waste; any water in the waste solution remains in the solidified product. Therefore, the solidified product has a larger volume than the original waste solution. Also, water, including groundwater, can leach compounds out of cement over time and cement is naturally porous, so the cement-stabilized product must be stored in leak-proof containers to prevent leaching.
Glass is the most long-term environmentally acceptable waste form. Glass is stable and extremely durable. Moreover, the hazardous species are chemically bonded in the glass structure, forming a substantially nonleachable composition. A number of vitrification processes for hazardous wastes have been described. Wheeler (U.S. Pat. No. 4,820,325) stabilizes toxic waste using a glass precursor material such as diatomaceous earth mixed with a compatible glass precursor material such as soda ash, lime (CaO) and alumina. The normally leachable toxicant becomes fixed within the glass when the mixture is vitrified. Hayashi, et al. (U.S. Pat. No. 4,725,383) add ZnO, or a mixture of ZnO with Al.sub.2 O.sub.3 and/or CaO, to a radioactive sodium borate waste solution. The resulting mixture is dehydrated and melted to produce a vitrified solid solution. Schulz, et al. (U.S. Pat. No. 4,020,004) vitrify radioactive ferrocyanide compounds by fusion with sodium carbonate (Na.sub.2 CO.sub.3) and a mixture of basalt and B.sub.2 O.sub.3, or silica (SiO.sub.2) and lime (CaO).
As discussed above, solidification of these and other wastes by glassification or vitrification is known, and generally involves combining glass forming compounds and/or natural rock, such as basalts or nepheline syenite, with the waste materials, and melting this mixture at temperatures sufficient to vitrify the mixture and immobilize the waste species in the resulting glass. The waste materials become dissolved in the melt and atomically bonded to the glass matrix that forms when the melt is cooled.
However, vitrification processes are not perfect, and problems are sometimes experienced due to the often limited solubility of the waste materials in the melt during glassification and/or due to the volatility of some or all of the waste species at the relatively high temperatures reached during the vitrification. Jantzen, "Systems Approach to Nuclear Waste Glass Development," J. Non-Cryst. Solids, 84 (1-3), 215-225, 1986. As a result, a glass forming additive that lowered the melting temperature of the mixture of waste and glass formers, thereby decreasing the amount or likelihood of waste volatilization, would be very desirable. In addition, a glass forming additive that increased the solubility of waste materials in the glass forming mixture, thereby increasing the amount of waste atomically bonded in the glass, increasing the waste loading capacity of the glass, and decreasing the disposal volume, would also be very desirable.
Lithia (Li.sub.2 O) has been disclosed to accelerate the dissolution of sand grains and increase melt rate. R. M. Wiker, Glass Industry, 37, 28, (1956). Lithia has also been disclosed to have a lower volatility than soda or potash at equal molar concentrations at 1400.degree. C. Volf, The Chemical Approach to Glass, 1984. Lithia has been disclosed to increase the viscosity of glass at low temperatures, N. W. Taylor et al., J. Amer. Ceram. Soc., 20, p. 296 (1937) and 24, p. 103 (1942), and to decrease the viscosity of glass at high temperatures, G. Heidtkamp et al., Glastechnick Bericht, 14, p. 99 (1936), as compared to soda (Na.sub.2 O) and potash (K.sub.2 O) containing glasses. Lithia is also disclosed to increase the modulus of elasticity in glasses compared to soda and potash. The mobile Li.sup.+ ion has also been disclosed to facilitate transmission of electric current, so that glasses containing Li.sup.+ melt at a faster rate in Joule heated melters than in gas fired commercial melters. Small amounts of Li.sup.+ have been disclosed to improve the electrical resistance during glass melting, decrease glass density (since Li.sup.+ is a light atomic weight element), and impart a low coefficient of thermal expansion to the glass. Lithia is used widely in glass ceramics, such as CHEMCOR and CORNINGWARE (Corning). Volf, The Chemical Approach to Glass, 1984.
Despite all of this, lithia is not considered to be a conventional glass component, and is used only for highly specialized applications in the commercial glass industry. Lithia is used in amounts in the range of about 0.7 to about 1.5 wt % to improve meltability in sealing glasses, used, e.g., to seal tungsten metal to glass or to seal different types of glass together. Lithia is also used in amounts of about 0.25 wt % to improve meltability in glass for discharge tubes and large mirrors of astronomical telescopes. Volf, The Chemical Approach to Glass, 1984.
One of the reasons for the failure of the art to use lithia more extensively in glassmaking is that the production of commercial container glasses, e.g., commercial bottle glass, and the production of commercial window glass typically does not involve the use of glass forming additives to lower the melting temperature of the glass. The melting point of these glasses is not of great concern because there are no hazardous species to volatilize, and they are routinely formulated to melt at temperatures of around 1300.degree. C. to 1400.degree. C. Lithia in particular is typically avoided in the commercial glass industry because it can cause undesirable effects, such as phase separation. Phase separation results from inhomogeneities in the glass which form regions of liquid-liquid immiscibility at higher temperatures that are retained when the glass cools. Phase separation can result in regions that are quite small and dispersed throughout the glass, and phase separated regions can have different optical and durability properties. W. Vogel, Chemistry of Glass, pp. 69-95, American Ceramic Society, 1985. These factors can combine to make phase separated container or window glass undesirably opaque. Schott Guide to Glass.
In addition, it is often considered undesirable to mix different alkali oxides in commercial glasses due to the "mixed alkali" effect, i.e., a significant reduction in the diffusion coefficient of the original alkali ion due to the presence of the second alkali ion, irrespective of the relative size of the alkali ions. M. Hara, Ion-channel match in mixed alkali glasses, J. Non-Cryst. Solids, 131 (1991). This effect can result in abnormal or nonlinear properties as a function of composition for a wide range of properties, including electrical conductivity of the melt, an important parameter in electric or Joule heated melters.
Jantzen, U.S. Pat. No. 5,102,439, disclose a borosilicate waste glass having about 5 wt % Li.sub.2 O produced in a melter operating at 1160.degree. C., but does not suggest that the presence of lithia has any appreciable effect on the melting point of the glass. To the contrary, the nonbridging oxygen equations described by Jantzen do not indicate that lithia has any properties that differentiate it from other alkali metal oxides.
Lead based paint and other coatings, as discussed above, is a form of hazardous waste subject to EPA control and regulation. Kumar et al., U.S. Pat. No. 5,292,375, disclose a process for removal of lead based paint without generation of airborne particles of hazardous waste, and without the need to control lead-containing water solutions. In the process of Kumar et al., particles of a glass mixture are flame sprayed onto the coated surface to form an overlying layer of glass material. As this layer cools, it spalls and separates from the coated structure, taking at least some of the lead based coating with it. This can be repeated as necessary to remove the lead based coating from the underlying structure. The spalled glass fragments can then be collected, remelted, tested for leachability and other regulatory compliance, and disposed of in a landfill. In general, the glass fragments obtained by the Kumar et al. process pass EPA regulations, while the material generated by sandblasting lead based paint will not. The glasses disclosed by Kumar et al. did not contain lithia or ferric oxide, and Kumar et al. make no suggestion to use these compounds. However, it has been disclosed that the spray properties (fluidity) and decreased retention of lead in the structure can be enhanced by the presence of about 2 wt % lithia and about 12.3 wt % ferric oxide. Marra et al., Glass Composition Development for a Thermal Spray Vitrification Process, Ceramic Trans. vol. 72, pp. 419-426, Am. Cer. Soc. (1996).
It is an object of the present invention to provide a method for decreasing the melting point of glass compositions used to immobilize hazardous, radioactive, or mixed waste, so that hazardous or radioactive species are less likely to volatilize during the melting process.
It is another object of the present invention to provide a method for increasing the solubility of hazardous or radioactive species in glass compositions for immobilizing them, and to increase the waste loadings of waste glass immobilization systems.
It is another object of the present invention to provide a process suitable for disposal of radioactive and hazardous waste, such as waste sludges and supernates, spent filter aids, incinerator ash, mixtures of incinerator ash and incinerator blowdown, geological mine tailings, asbestos, loaded ion exchange resins, contaminated soils, and lead based paint and other coatings.
It is another object of the present invention to provide glass compositions that have significantly decreased melting temperatures as compared to those compositions generally used for waste stabilization, including borosilicate glass, soda-lime-silica glass, soda-baria-silica glass, and soda-magnesia-silica glass.
It is another object of the present invention to provide glass compositions whose preparation reduces the occurrence of melter off-gas line pluggage, allows the use of less expensive and more robust melter designs, and provides longer melter design life.
It is another object of the invention to provide waste glass production processes and compositions that lower vitrification temperatures, increase waste loadings, provide for large waste volume reductions, and produce durable glasses suitable for waste stabilization and capable of meeting regulatory guidelines.